The neuropathological correlates of Alzheimer's disease (AD) include amyloid-β (Aβ) plaques and neurofibrillary tangles. To study the interaction between Aβ and tau and their effect on synaptic function, we derived a triple-transgenic model (3×Tg-AD) harboring PS1_M146V, APP_Swe, and tau_P301L transgenes. Rather than crossing independent lines, we microinjected two transgenes into single-cell embryos from homozygous PS1_M146V knockin mice, generating mice with the same genetic background. 3×Tg-AD mice progressively develop plaques and tangles. Synaptic dysfunction, including LTP deficits, manifests in an age-related manner, but before plaque and tangle pathology. Deficits in long-term synaptic plasticity correlate with the accumulation of intraneuronal Aβ. These studies suggest a novel pathogenic role for intraneuronal Aβ with regards to synaptic plasticity. The recapitulation of salient features of AD in these mice clarifies the relationships between Aβ, synaptic dysfunction, and tangles and provides a valuable model for evaluating potential AD therapeutics as the impact on both lesions can be assessed.